Several of the many desirable characteristics sought in a friction material are low cost, high wear resistance, high heat resistance, high coefficients of friction, consistent coefficients of friction over a wide heat and load range and time periods, and close or identical static and dynamic coefficients of friction. Wide. differences in static and dynamic coefficients of friction are believed by some to cause or exist conincident with the phenomenon of stick-slip or chatter in clutches and brakes. The friction material of the present invention is relatively inexpensive, exhibits a high level of these desirable characteristics, and eliminates or virtually eliminates chatter when used in wet clutches and brakes.
A form of the friction material of the present invention is illustrated for use in a locking differential of the type disclosed in U.S. Pat. No. 3,831,462, issued to Jerry F. Baremor and a limited slip differential of the type disclosed in U.S. Pat. No. 3,624,717, issued to Richard K. Brubaker. The Baremor differential includes a multiple disc friction clutch which is applied to retard relative rotation between driven axles only when the relative rotation exceeds a predetermined level. The Brubaker differential includes a pair of multiple disc friction clutches which are continuously applied be relatively light spring forces and by parting forces acting between the differential side gears and pinions. These types of differentials, broadly referred to as locking and limited slip differentials, have long suffered from the annoying problem of clutch chatter; this problem is particularly well-known in the limited slip differentials.
The friction material of the present invention is readily traced to the Aerospace Industry. Aerospace needs, particularly in the past three decades, have fostered the development of many exotic materials which are commerically successful in the aerospace industry but which fail to achieve commercial success in other industries due to the high cost, e.g., the automotive industry. One such material has been high carbon density composite material produced by chemical vapor deposition (CVD) of carbon or graphite on a precursor cloth substrate. The substrate may be a felt or a woven fabric composed of carbon or graphite.
Carbon materials produced by CVD are generally referred to as pyrolytic carbon or graphite. Herein, carbon and graphite materials produced by CVD on a carbon or graphite cloth will be referred to as pyrolytic carbon composite material unless explicitly stated otherwise.
Since the 1950's, articles made of high density pyrolytic carbon composite materials have been used in aerospace applications requiring high strength at relatively high operating temperatures. Examples of such articles include radiation shields and rocket nozzles. These articles have been formed of multiple layers of a preshaped cloth substrate, e.g., 20 or more layers, CVD infiltrated for hundreds of hours to as high a density as could be obtained. The finished articles resemble a solid block of carbon in that they are relatively inflexible, are impervious to light, have the meshes of the cloth substrate completely filled or virtually filled with carbon deposited by CVD, and have a relatively smooth outer surface as free of pores as possible.
Since the late 1960's or early 1970's these same high density carbon composite materials have been recognized as excellent friction materials for dry aircraft brakes when the engaged friction surfaces are both formed of the composite material, i.e., composite material running on composite material. Herein a dry brake or clutch is defined as a brake or clutch wherein the friction surfaces are dry and a wet brake or clutch is defined as a brake or clutch wherein the friction surfaces are lubricated or cooled by direct contact with a liquid. Unsatisfactory levels of chatter were obtained with dry brakes having one friction surface of the composite material and the other of steel or iron.
In spite of the excellent results obtained in the dry brake use of the pyrolytic carbon composite material, the material has not found widespread use in clutches and brakes outside the aerospace industry. One reason for this lack of commercial success outside of the aerospace industry is believed to be related to, if not solely due to, the material's high cost compared to other friction materials. A substantial portion of this high cost is due to the great many hours (both total and labor hours) required to successfully CVD infiltrate the multiple layers of cloth substrate to the high carbon density believed necessary for use of the material as a friction material. Further, since satisfactory results could only be obtained by running composite material against composite material, the amount of material required was effectively doubled.
U.S. Pat. No. 3,991,248, issued to Dieter W. Bauer in 1975, provides one example of the time-consuming process for CVD infiltrating multiple layers of cloth substrate to a high carbon density. As taught in the Bauer patent, the substrate may consist of multiple layers of a rayon precursor graphite cloth. The cloth layers are abraded on both sides to raise the nap of the cloth, cut into a desired shape, and stacked in layers in a compacting fixture. The cloth is then CVD infiltrated for as much as several hundred hours in a batch furnace. The CVD carbon infiltrated between the layers provides a matrix penetrated by the nap fibers to increase the layer-to-layer bond or shear strength between the layers. More specifically, as shown in Example 1, Sample 5, of the Bauer patent, twenty-five layers of abrated cloth are cut into annular discs having an O.D. of 3.650 inches (9.271 cm) and an I.D. of 0.750 inches (1.905 cm) and then stacked in the fixture of FIG. 1. The layers are compacted or compressed by the fixture to a total substrate thickness of 0.350 inches and then CVD infiltrated for ten hours and then for 160 hours. The ten-hour infiltration run, according to Table 2, increased the density from 0.717 grams per cubic centimeter to 1.070 grams per cubic centimeter, a carbon density increase of 0.0253 grams per cubic centimeter per hour of infiltration time. The 160-hour infiltration run increased the density from 1.070 grams per cubic centimeter to 1.590 grams per cubic centimeter, a carbon density increase of only 0.00325 grams per cubic centimeter per hour of infiltration time. Between infiltration runs the partially infiltrated substrate was removed from the fixture and machined on all of its sides to remove bottleneck pores which prevent or impede further infiltration. After machining, the substrate was ultrasonically cleaned in acetone prior to continued infiltration.
U.S. Pat. No. 3,897,582, issued to Eugene L. Olcott in 1975, provides a second example of a process for making a pyrolytic carbon composite friction material for an aircraft brake. The material is formed by progressively positioning a continuous or a plurality of continuous refractory strands on a heated mandrel and simultaneously pyrolyzing pyrolysis gases onto the strand(s) at about the point of positioning contact of the strand(s) on the mandrel or on previously positioned strands to nucleate pyrolytic graphite. One object of this process is to provide a material having few or a minimum number of voids or pores which substantially reduce the density of the material.
The pyrolytic carbon composite materials produced by the processes of U.S. Pat. Nos. 3,991,248 and 3,897,582, like the composite materials produced for radiation shields and rocket nozzles, resemble solid blocks of carbon in that they are relatively inflexible, impervious to light, have few, if any, through pores, and have relatively small surface pores.
Applicant has discovered that pyrolytic carbon composite material having multiple layers of cloth substrate highly densified by CVD infiltration provides chatter-free results when run against a metal, such as steel or iron, in a wet clutch or brake. This discovery reduces the amount of composite material required for a given clutch or brake and therefore reduces costs. The material, when used in wet clutches and brakes, seems to be insensitive to the type of lubricating or cooling oil used. For example, many friction materials are relatively chatter-free when used with extreme high pressure oils but chatter unacceptably when used with light pressure oils.
Applicant has further discovered, that in certain wet clutch and brake applications requiring relatively thin amounts of friction material in the range of 0.080 to 0.010 inches, the pyrolytic carbon composite material may be formed of a single porous layer of cloth substrate or two or three interwoven porous layers of fabric CVD infiltrated for relatively short periods of time compared to the infiltration time periods taught in the Bauer patent. In fact excellent results have been obtained with single layers of square weave fabrics CVD infiltrated for several minutes by a process taught in U.S. Pat. No. 3,944,686, issued to Robert W. Froberg.
The Froberg patent, which is incorporated herein by reference, discloses a process for making electrodes for use in fuel cells. The process, unlike the batch process in the Bauer patent, is a continuous process wherein relatively long strips of porous precursor cloth (felt or woven fabric) continuously moves through a CVD infiltrating furnace in conveyor belt fashion. When single layers of woven fabric are infiltrated by this process, or for that matter quickly infiltrated, the finished material is still porous and is readily recognized as a fabric, is relatively flexible for a given thickness compared to the highly densified material of Bauer U.S. Pat. No. 3,991,248, has open or unfilled meshes (i.e., through pores), and is pervious to light. The specific process for making this quickly infiltrated material forms no part of the present invention, e.g., the material may be made in batch process furnaces or continuous process furnaces.
A second process for making such electrode material is disclosed in British Patent Specification No. 1,455,891, published Nov. 17, 1976. The electrode material of this specification is made by batch furnace process but is otherwise the same as the material of Froberg U.S. Pat. No. 4,944,686. That is, both electrode materials are formed from a porous substrate on precursor membrane of carbon fibers. The precursor membrane may be characterized as having an open porosity, a mesh or open mesh, or through pores; herein these are synonymous terms which are collectively included in the term mesh. Both electrode materials are lightly densified or coated with pyrolytic carbon relative to the material of Bauer U.S. Pat. No. 3,991,248. The final density of this lightly densified material is of course a function of the bulk density of the precursor (i.e., compactness of the carbon fibers) and the amount of pyrolytic carbon deposited by chemical vapor deposition. The lightly densified material is, however, structurally distinguished from the highly densified material in that it retains a substantial amount of the porosity of the starting precursor relative to the highly densified material and retains a substantial amount of the mesh of the starting precursor, whereas the mesh of the highly densified material is virtually closed by pyrolytic carbon. Further, the lightly densified material retains a substantial amount of the compressibility of the starting pecursor, whereas the precursor of the highly densified material is rigidly incapsulated by a dense pyrolytic carbon matrix which is intended to increase structural strength and render the densified material virtually incompressibile when used as a friction material.
Tests of this relatively thin and quickly infiltrated material have provided excellent results in wet clutches and brakes having the material running against steel and against itself. Further, when formed of a single layer of woven fabric, this material seems to exhibit improved bonding characteristics when it is adhesively bonded to a support member such as a steel disc used in a limited slip differential.